The present invention relates generally to telecommunications and data storage and more particularly to a method for accurate determination of differential distance in paired optical fibers.
A computer system generally contains a storage system in which data of the computer system is stored. Some storage systems have a function of copying data between two storage systems. The remote copy function is to copy data of one storage system to the other storage system by transferring the data via a network. Recent storage systems increasingly employ disaster recovery systems that use remote copy. A disaster recovery system remote copies data from a primary site to a remote or secondary site to enable the business to continue after a failure, for instance caused by a natural disaster.
A disaster recovery system in some cases places the secondary site several hundreds of kilometers away from the primary site. Such disaster recovery systems rely on the relative accuracy of clock time between various nodes in these multi-site networks. Accordingly, systems or methods are needed to maintain clock accuracy in such applications. For example, in some modem systems, timing synchronization is maintained between servers via links capable of supporting a Server Time Protocol (STP). In order for STP to perform within the overall parameters of the system architecture, the differential delay between the transmit and receive direction of a link supporting the STP protocol must be maintained within a certain threshold. Total transit time includes not only the propagation of time through the optical system, but also delays in the electronics that drive the optical signal; therefore, fiber lengths must be matched. To ensure correct time synchronization, the end-to-end lengths of the transmit and receive fibers within an individual external time reference must be within a predetermined value.
Optical low differential delay systems exist today, but with limited distances (10 km). There is a desire to extend the reach of these systems to distances up to hundreds of kilometers using dense wavelength division multiplexers (DWMDs). However, differential delay is a system parameter which must be maintained within a range to prevent data corruption through undetected loss of synchronization. Therefore it is desirable to have a tool for measuring differential delay in the field.